Bad news for those inclined to toke and drive: A Canadian study shows that smoking pot within three hours of getting behind the wheel nearly doubles your chance of being involved in a car crash.

Associate Professor Mark Asbridge and his colleagues from Dalhousie University in Halifax pored over data from almost 50,000 traffic incidents, specifically identifying cases where drivers had smoked up before driving but had not been drinking or taking other drugs. While accidents tended to be less common and less serious than crashes resulting from drunk driving, the increase in incident frequency was obvious.

The authors note that the study isn't exhaustive — among other things, there are still questions about how much THC has to be in someone's system before the bummer risk becomes substantial — but it definitely points to the increased risks of driving with anything less than a clear head.